Better Maps

Navigating the wiring of the nervous system, as with any convoluted roadway, requires a good map. Neuroscientists have been tracing the paths of neurons since the late 1800s, when Spaniard Santiago Ramón y Cajal stained brain tissue and provided evidence that the nervous system was made up of separate, yet interconnected cells.

More than a decade ago, HHMI investigator Liqun Luo and then-postdoctoral fellow Tzumin Lee, who is now a group leader at HHMI’s Janelia Farm Research Campus, improved on traditional stains by developing MARCM—shorthand for mosaic analysis with a repressible cell marker. The method provides a clearer view of cells. Typical staining methods color cells randomly and rampantly. But Luo’s approach uses genetic tricks to light up only certain cells. That means a researcher can make visible only the cells that she is interested in seeing.

To pull it off, Luo reengineers cells with a set of genes. One gene manufactures a fluorescent protein visible under a microscope. But that gene turns on only when a genetic exchange occurs nearby. Luo introduces other genes that control such genetic exchanges but engineers them so they work only in cells he wants to see. The end result: a select group of cells that can be located and traced through the brain.

The approach offers several advantages. First, the fluorescent labeling can be targeted to specific types of cells. Second, the labeling is sparse, so individual cells are distinct under the microscope. In addition, researchers can alter a gene and know by the fluorescent label exactly which cell carries the change.

Luo’s group has used the approach to map how fly brains are wired during development (see “Nerve Cell Navigation,” HHMI Bulletin, August 2008) as well as to chart interneuron connections in the olfactory system (see main story). Many other groups around the world are now using MARCM to study neuron wiring in the fly brain, and Luo’s original paper describing it is his most cited. Luo’s group has adapted the technique to map neurons in mice as well. The tool will help ensure that researchers keep their bearings as they make sense of a brain’s superhighways, local roads, and intersections.